An antenna is an electrical device that converts electric power into radio waves, and vice versa. It is usually used with a radio transmitter in transmission or a radio receiver in reception. In transmission, the radio transmitter supplies an electric current oscillating at a radio frequency (e.g., a high frequency alternating current (AC)) to the antenna's terminals. The antenna then radiates the energy from the current as electromagnetic waves (radio waves). In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals that is applied to the receiver to be amplified. Antennas are components of most equipment that uses radio. They are used in systems such as radio broadcasting, broadcast television, two-way radio, communications receivers, radar, cell phones, and satellite communications.
In conjunction with an antenna, a filter may be used on signals sent or received by the antenna. One such filter may comprise a band-stop filter. In signal processing, a band-stop filter (i.e., band-rejection filter) is a filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. A band-stop filter is the opposite of a band-pass filter. A notch filter is a band-stop filter with a narrow stopband. A notch filter may be used in conjunction with an antenna.